For experimental birds, a permanent marker was used to blacken the white tips of the three outer retrices. Avian geophagy and soil characteristics in southeastern Peru. Resources in your library. Villi are projections from the intestinal wall that increase the amount of surface area available for absorption. The four segments of the duodenum are as follows starting at the stomach, and moving toward the jejunum: Food digestion physiology varies between individuals and upon other factors such as the characteristics of the food and size of the meal, and the process of digestion normally takes between 24 and 72 hours. Within 14 days, they showed a doubling of the size of their gizzards.
The mucosa is made up of:. The mucosae are highly specialized in each organ of the gastrointestinal tract to deal with the different conditions.
The most variation is seen in the epithelium. The submucosa consists of a dense irregular layer of connective tissue with large blood vessels, lymphatics, and nerves branching into the mucosa and muscularis externa. It contains the submucosal plexus , an enteric nervous plexus , situated on the inner surface of the muscularis externa. The muscular layer consists of an inner circular layer and a longitudinal outer layer.
The circular layer prevents food from traveling backward and the longitudinal layer shortens the tract. The layers are not truly longitudinal or circular, rather the layers of muscle are helical with different pitches.
The inner circular is helical with a steep pitch and the outer longitudinal is helical with a much shallower pitch. Whilst the muscularis externa is similar throughout the entire gastrointestinal tract, an exception is the stomach which has an additional inner oblique muscular layer to aid with grinding and mixing of food.
The muscularis externa of the stomach is composed of the inner oblique layer, middle circular layer and outer longitudinal layer. Between the circular and longitudinal muscle layers is the myenteric plexus.
Activity is initiated by the pacemaker cells, myenteric interstitial cells of Cajal. The gut has intrinsic peristaltic activity basal electrical rhythm due to its self-contained enteric nervous system. The rate can be modulated by the rest of the autonomic nervous system. The coordinated contractions of these layers is called peristalsis and propels the food through the tract. Food in the GI tract is called a bolus ball of food from the mouth down to the stomach.
After the stomach, the food is partially digested and semi-liquid, and is referred to as chyme. In the large intestine the remaining semi-solid substance is referred to as faeces. The outermost layer of the gastrointestinal tract consists of several layers of connective tissue. Intraperitoneal parts of the GI tract are covered with serosa. These include most of the stomach , first part of the duodenum , all of the small intestine , caecum and appendix , transverse colon , sigmoid colon and rectum.
In these sections of the gut there is clear boundary between the gut and the surrounding tissue. These parts of the tract have a mesentery. Retroperitoneal parts are covered with adventitia. They blend into the surrounding tissue and are fixed in position. For example, the retroperitoneal section of the duodenum usually passes through the transpyloric plane. These include the esophagus , pylorus of the stomach, distal duodenum , ascending colon , descending colon and anal canal.
In addition, the oral cavity has adventitia. Specific proteins expressed in the stomach and duodenum involved in defence include mucin proteins, such as mucin 6 and intelectin Finally, transit through the colon takes 12 to 50 hours with wide variation between individuals. The gastrointestinal tract forms an important part of the immune system. There are additional factors contributing to protection from pathogen invasion. For example, low pH ranging from 1 to 4 of the stomach is fatal for many microorganisms that enter it.
Beneficial bacteria also can contribute to the homeostasis of the gastrointestinal immune system. For example Clostridia , one of the most predominant bacterial groups in the GI tract, play an important role in influencing the dynamics of the gut's immune system. This is due to the production of short-chain fatty acids during the fermentation of plant-derived nutrients such as butyrate and propionate.
Basically, the butyrate induces the differentiation of Treg cells by enhancing histone H3 acetylation in the promoter and conserved non-coding sequence regions of the FOXP3 locus, thus regulating the T cells , resulting in the reduction of the inflammatory response and allergies. The large intestine hosts several kinds of bacteria that can deal with molecules that the human body cannot otherwise break down.
These bacteria also account for the production of gases at host-pathogen interface , inside our intestine this gas is released as flatulence when eliminated through the anus. However the large intestine is mainly concerned with the absorption of water from digested material which is regulated by the hypothalamus and the re absorption of sodium , as well as any nutrients that may have escaped primary digestion in the ileum.
Health-enhancing intestinal bacteria of the gut flora serve to prevent the overgrowth of potentially harmful bacteria in the gut. These two types of bacteria compete for space and "food," as there are limited resources within the intestinal tract. Enzymes such as CYP3A4 , along with the antiporter activities, are also instrumental in the intestine's role of drug metabolism in the detoxification of antigens and xenobiotics.
There are many diseases and conditions that can affect the gastrointestinal system, including infections , inflammation and cancer. Various pathogens can cause gastroenteritis an inflammation of the stomach and small intestine. These can include those organisms that cause foodborne illnesses. Gastroenteritis is the most common disease of the GI tract. Diverticular disease is a condition that is very common in older people in industrialized countries.
It usually affects the large intestine but has been known to affect the small intestine as well. Diverticulosis occurs when pouches form on the intestinal wall. Once the pouches become inflamed it is known as diverticulitis.
Inflammatory bowel disease is an inflammatory condition affecting the bowel walls, and includes the subtypes Crohn's disease and ulcerative colitis. While Crohn's can affect the entire gastrointestinal tract, ulcerative colitis is limited to the large intestine. Crohn's disease is widely regarded as an autoimmune disease.
Although ulcerative colitis is often treated as though it were an autoimmune disease, there is no consensus that it actually is such. Functional gastrointestinal disorders the most common of which is irritable bowel syndrome.
Functional constipation and chronic functional abdominal pain are other functional disorders of the intestine that have physiological causes, but do not have identifiable structural, chemical, or infectious pathologies. Gastrointestinal surgery can often be performed in the outpatient setting.
In the United States in , operations on the digestive system accounted for 3 of the 25 most common ambulatory surgery procedures and constituted 9. Various methods of imaging the gastrointestinal tract include the upper and lower gastrointestinal series:. Animal intestines have multiple uses. From each species of livestock that is a source of milk , a corresponding rennet is obtained from the intestines of milk-fed calves.
Pig and calf intestines are eaten, and pig intestines are used as sausage casings. This releases carbohydrates, protein, fat, and various vitamins and minerals for absorption into the body. In most vertebrates , digestion is a multistage process in the digestive system, starting from ingestion of raw materials, most often other organisms.
Ingestion usually involves some type of mechanical and chemical processing. Digestion is separated into four steps:. Underlying the process is muscle movement throughout the system through swallowing and peristalsis. Each step in digestion requires energy, and thus imposes an "overhead charge" on the energy made available from absorbed substances.
Differences in that overhead cost are important influences on lifestyle, behavior, and even physical structures. Examples may be seen in humans, who differ considerably from other hominids lack of hair, smaller jaws and musculature, different dentition, length of intestines, cooking, etc.
The major part of digestion takes place in the small intestine. The large intestine primarily serves as a site for fermentation of indigestible matter by gut bacteria and for resorption of water from digests before excretion. In mammals , preparation for digestion begins with the cephalic phase in which saliva is produced in the mouth and digestive enzymes are produced in the stomach. Mechanical and chemical digestion begin in the mouth where food is chewed , and mixed with saliva to begin enzymatic processing of starches.
The stomach continues to break food down mechanically and chemically through churning and mixing with both acids and enzymes. Absorption occurs in the stomach and gastrointestinal tract , and the process finishes with defecation. The human gastrointestinal tract is around 9 meters long. Food digestion physiology varies between individuals and upon other factors such as the characteristics of the food and size of the meal, and the process of digestion normally takes between 24 and 72 hours.
Digestion begins in the mouth with the secretion of saliva and its digestive enzymes. Food is formed into a bolus by the mechanical mastication and swallowed into the esophagus from where it enters the stomach through the action of peristalsis. Gastric juice contains hydrochloric acid and pepsin which would damage the walls of the stomach and mucus is secreted for protection. In the stomach further release of enzymes break down the food further and this is combined with the churning action of the stomach.
The partially digested food enters the duodenum as a thick semi-liquid chyme. In the small intestine, the larger part of digestion takes place and this is helped by the secretions of bile , pancreatic juice and intestinal juice. The intestinal walls are lined with villi , and their epithelial cells is covered with numerous microvilli to improve the absorption of nutrients by increasing the surface area of the intestine.
In the large intestine the passage of food is slower to enable fermentation by the gut flora to take place.
Here water is absorbed and waste material stored as feces to be removed by defecation via the anal canal and anus. Different phases of digestion take place including: The cephalic phase occurs at the sight, thought and smell of food, which stimulate the cerebral cortex.
Taste and smell stimuli are sent to the hypothalamus and medulla oblongata. After this it is routed through the vagus nerve and release of acetylcholine. Acidity in the stomach is not buffered by food at this point and thus acts to inhibit parietal secretes acid and G cell secretes gastrin activity via D cell secretion of somatostatin. The gastric phase takes 3 to 4 hours. It is stimulated by distension of the stomach, presence of food in stomach and decrease in pH. Distention activates long and myenteric reflexes.
This activates the release of acetylcholine , which stimulates the release of more gastric juices. As protein enters the stomach, it binds to hydrogen ions, which raises the pH of the stomach.
Inhibition of gastrin and gastric acid secretion is lifted. This triggers G cells to release gastrin , which in turn stimulates parietal cells to secrete gastric acid.
Gastric acid is about 0. Acid release is also triggered by acetylcholine and histamine. The intestinal phase has two parts, the excitatory and the inhibitory. Partially digested food fills the duodenum. This triggers intestinal gastrin to be released. Enterogastric reflex inhibits vagal nuclei, activating sympathetic fibers causing the pyloric sphincter to tighten to prevent more food from entering, and inhibits local reflexes.
Protein digestion occurs in the stomach and duodenum in which 3 main enzymes, pepsin secreted by the stomach and trypsin and chymotrypsin secreted by the pancreas, break down food proteins into polypeptides that are then broken down by various exopeptidases and dipeptidases into amino acids.
The digestive enzymes however are mostly secreted as their inactive precursors, the zymogens. For example, trypsin is secreted by pancreas in the form of trypsinogen , which is activated in the duodenum by enterokinase to form trypsin. Trypsin then cleaves proteins to smaller polypeptides. Digestion of some fats can begin in the mouth where lingual lipase breaks down some short chain lipids into diglycerides.
However fats are mainly digested in the small intestine. In humans, dietary starches are composed of glucose units arranged in long chains called amylose, a polysaccharide.
During digestion, bonds between glucose molecules are broken by salivary and pancreatic amylase , resulting in progressively smaller chains of glucose. This results in simple sugars glucose and maltose 2 glucose molecules that can be absorbed by the small intestine.
Lactase is an enzyme that breaks down the disaccharide lactose to its component parts, glucose and galactose.
Glucose and galactose can be absorbed by the small intestine. Approximately 65 percent of the adult population produce only small amounts of lactase and are unable to eat unfermented milk-based foods. This is commonly known as lactose intolerance. Lactose intolerance varies widely by ethnic heritage; more than 90 percent of peoples of east Asian descent are lactose intolerant, in contrast to about 5 percent of people of northern European descent. Sucrase is an enzyme that breaks down the disaccharide sucrose , commonly known as table sugar, cane sugar, or beet sugar.
Sucrose digestion yields the sugars fructose and glucose which are readily absorbed by the small intestine. Some nutrients are complex molecules for example vitamin B 12 which would be destroyed if they were broken down into their functional groups.
To digest vitamin B 12 non-destructively, haptocorrin in saliva strongly binds and protects the B 12 molecules from stomach acid as they enter the stomach and are cleaved from their protein complexes.
After the B 12 -haptocorrin complexes pass from the stomach via the pylorus to the duodenum, pancreatic proteases cleave haptocorrin from the B 12 molecules which rebind to intrinsic factor IF. These B 12 -IF complexes travel to the ileum portion of the small intestine where cubilin receptors enable assimilation and circulation of B 12 -IF complexes in the blood. There are at least five hormones that aid and regulate the digestive system in mammals. There are variations across the vertebrates, as for instance in birds.
Arrangements are complex and additional details are regularly discovered. For instance, more connections to metabolic control largely the glucose-insulin system have been uncovered in recent years. Digestion is a complex process controlled by several factors. In the mouth, pharynx and esophagus, pH is typically about 6. Saliva controls pH in this region of the digestive tract. Salivary amylase is contained in saliva and starts the breakdown of carbohydrates into monosaccharides.
Most digestive enzymes are sensitive to pH and will denature in a high or low pH environment. The stomach's high acidity inhibits the breakdown of carbohydrates within it. This acidity confers two benefits: Herons use their bills to spear small fish and amphibians.
Macaws use their strong hook-like bills to feed on nuts. A kingfisher using its bill to capture prey! Great Egret slow motion. Red-tailed Hawk eating a red squirrel. Sea-anchor 'soaring' by storm-petrels -- As described by Pennycuick , storm-petrels have a unique method for picking up food from the water's surface.
They glide into the wind with their body clear of the surface, but with their webbed feet in the water, helping to 'anchor' them. Aerodynamic or air drag on on wings friction drag and body parasitic drag push them backward through the water so that the feet tend to slowly move backward through the water, and that creates forward-directed hydrodynamic or water drag. When this water drag balances the aerodynamic air drag, the storm-petrels remain in place, suspended above the water's surface at a height that allows them to pick small food items from the surface Photo source: The beak's outer shell is made of hexagonal keratin tiles cemented together with an organic glue and piled in several staggered layers.
They perch on sturdier portions of a branch and use their long beak to reach their food. So, the beak must be rigid enough to resist bending and twisting forces, but has to be light or the bird couldn't get off the ground.
The lightweight strength of the toucan's beak is due to a matrix of bony fibers and drum-like membranes sandwiched between an outer layer of keratin. The interior of the beak is rigid "foam" composed of bony fibers and drum-like membranes sandwiched between outer layers of keratin. Woodpecker shock-absorbing system -- Woodpeckers are known to drum hard woody surfaces of trees at a rate of 18 to 22 times per second with a deceleration of g humans can lose consciousness at g-forces as low as 4 to 6 g.
Woodpeckers have four structures that help absorb mechanical shock and prevent brain damage: A woodpecker's hyoid extends posteriorly from the floor of the oral cavity, goes behind the neck, divides into two bands, goes around the back of the skull, and inserts at the front of the skull. This allows woodpeckers to extend their tongues well beyond the tip of the beak when foraging, but this arrangement also helps distribute mechanical vibrations when drumming.
The spongy bone is thought to evenly distribute mechanical vibrations before they propogate to the brain. Yoon and Park Yoon and Park built a shock-absorbing device with mechanical 'woodpecker' analogues that consisted of glass beads embedded in a steel-encased aluminum cylinder.
They shot it with an airgun and found that the new device protected its contents electronic equipment at forces up to 60, g. To evenly distribute load and cut down on vibration, like the hyoid, they added a rubber layer. Woodpecker's head inspires shock absorbers. The unequal length of the upper and lower parts of woodpecker beaks the lower being longer directs the force of impact downwards, away from the brain, when it hits the tree red coloration indicates the greatest force or stess; blue indicates the least force or stress.
Time after impact proceeds from upper left 0. The hyoid of woodpeckers loops over top of the skull to completely surround their skulls. The hyoid helps direct the stress of impact below and around the skull and brain and also acts like a 'safety belt', helping to keep the brain in place. It is the movement of the brain inside the skull during impact, more than the blow itself, that causes concussions.
If the brain is held in place, injury risks are greatly reduced. As in the above figure, time after impact proceeds from upper left to lower right Figures from Wang et al. Built to peck - Segment 2. Built to peck - Segment 4. Built to peck - Segment 5. Built to peck - Segment 6. Built to peck - Segment 7. The capillary ratchet mechanism. Surface tension transport of prey by feeding shorebirds: They peck at the surface, picking up droplets of water with prey inside.
Because their beaks point downward when feeding, gravity must be overcome to get those droplets from the tip of the bird's long beak to its mouth. This feeding strategy depends on surface tension. As the beak opens and closes, each movement propels the water droplet one step closer to the bird's mouth.
Specifically, when the beak closes, the drop's leading edge moves toward the mouth, while the trailing edge stays put. In this stepwise ratcheting fashion, the drop travels along the beak at a speed of about 1 meter per second.
The efficiency of the process, called the "capillary ratchet," depends on beak shape, and long, narrow beaks, like those of phalaropes, are best suited to this mode of feeding. Serin Serinus serinus with a seed positioned in its bill. Note how the tongue is used to hold the seed in position From: Their data provide the first detailed description of this highly specialized foraging technique.
They recorded no or a very low deceleration when Gannets entered the water, which underlines the remarkable streamlining of this large bird. Birds use their momentum to travel underwater at an average descent rate of 2. After chasing prey, birds developed an upward momentum before gliding passively back to the surface, making use of their buoyancy to complete the dive at the lowest possible energy cost.
Check the Gannet videos at ARKive. Aerial insectivores -- Swifts depend on flying efficiently and maintaining high speed. Hawking insectivores, like flycatchers , depend on perches located near prey, but they must be able to accelerate rapidly and be very maneuverable. Swallows combine these two strategies; they are fast, maneuverable and able to accelerate when necessary Warrick Although not part of the digestive system in an anatomical sense, some birds, like hawks and owls , use their feet and talons to capture prey.
Typically, raptor prey are killed by the talons of the contracting foot being driven into their bodies; if required, the hooked bill is used to kill prey being held by the talons. The raptor digital tendon locking mechanism -- Digital tendons form a mechanical-locking mechanism in many birds that must maintain a degree of grip force, including perching, hanging, tree-climbing, and raptorial species.
In raptors, powerful hindlimb muscles produce a strong grasp, and a tendon locking mechanism TLM helps sustain grip force. The components of the digital TLM include a 'textured' pad on the ventral surface of each flexor tendon that contains thousands of minute, rigid, well-defined projections called tubercles see figure below.
The neighboring portion of the surrounding tendon sheath contains a series of transversely running plicae folds that often have a proximal slant i. When the flexor tendons are pulled taut, and the digits flexed, the tubercle pad moves proximally over the stationary plicae on the sheath. When resistance to digital flexion is met, the locking elements intermesh and engage and the friction produced prevents slippage of the tendons.
This permits digital flexion to be maintained with little or no muscular involvement E inoder and Richardson Action of the avian digital TLM: This shows the movement of the talon a , flexor e and extensor d tendons, ungual phalanx b , and the movement of the ventrally located tubercle pad f relative to the stationary plicated sheath g and phalangeal bone c From: Einoder and Richardson Each raptor has a unique force production, along with a different time of activity, that would allow for a degree of prey specialization.
Great Horned Owl foot. B Great Horned Owl. The relation between rate of success and direction of movement for a food item that was pulled forward a , backward b and sideways c. Direction of prey progression — dotted arrow 1 , direction of owl flight — dashed arrow 2 , and direction to which the owl had to move its head or trunk — solid arrow 3. Owl picture from Knudsen Movement and direction of prey affect raptor success rate -- Shifferman and Eilam tested a novel idea, that rather than maximizing their distance from a predator during close-distance encounters, prey species are better off moving directly or diagonally toward the predator in order to increase the relative speed and confine the attack to a single available clashing point.
They used two tamed Barn Owls Tyto alba to measure the rate of attack success in relation to the direction of prey movement. A dead mouse or chick was used to simulate the prey, pulled to various directions by means of a transparent string during the owl's attack. This failure to catch prey that move sideways may reflect constraints in postural head movements in aerial raptors that cannot move the eyes but rather move the entire head in tracking prey. So far there is no evidence that defensive behavior in terrestrial prey species takes advantage of the above escape directions to lower rates of predator success.
However, birds seem to adjust their defensive tactics in the vertical domain by taking-off at a steep angle, thus moving diagonally toward the direction of an approaching aerial predator. These preliminary findings warrant further studies in Barn Owls and other predators, in both field and laboratory settings, to uncover fine predator head movements during hunting, the corresponding defensive behavior of the prey, and the adaptive significance of these behaviors.
Barred Owl primary - leading edge below and trailing edge above. The silent flight of owls -- Noise is generated by vortices produced when air flows over a bird's wing and larger vortices produce more noise. Wings with small saw-toothed projections vortex generators , like those on the leading edge of owl wings, generate many small vortices instead of large vortices and produces less aerodynamic noise.
In addition, the fringe feathers at the trailing edge of the wing with fewer hooklets at the ends of the barbs help to break up the sound waves that are generated as air flows over the top of their wings and forms downstream wakes, and the soft down feathers located elsewhere on the wings and legs of owls absorb the remaining sound frequencies above 2, hertz and make owls completely silent to their prey.
As a bonus, with high angles of attack and at slow speeds, vortex generators stick out of the stagnant air near the surface of the wing, and into the freely moving air outside the boundary layer.
This surface layer is typically quite thin, but dramatically reduces speed of the airflow towards the rear of the wing. The vortex generators mix the free stream with the stagnant air to get it moving again, providing considerably more airflow at the rear of the wing and helping to prevent stalling. This process is referred to as 're-energizing the boundary layer. Unpredictable predators -- The use of space by predators in relation to their prey is a poorly understood aspect of predator-prey interactions.
Classic theory suggests that predators should focus their efforts on areas of abundant prey, that is, prey hotspots, whereas game-theoretical models of predator and prey movement suggest that the distribution of predators should match that of their prey's resources. If, however, prey are spatially anchored to one location and these prey have particularly strong antipredator responses that make them difficult to capture with frequent attacks, then predators may be forced to adopt alternative movement strategies to hunt behaviorally responsive prey.
Roth and Lima examined the movement patterns of bird-eating Sharp-shinned Hawks Accipiter striatus in an attempt to shed light on hotspot use by predators. Their results suggest that these hawks do not focus on prey hotspots such as bird feeders but instead maintain much spatial and temporal unpredictability in their movements. Hawks seldom revisited the same area, and the few frequently used areas were revisited in a manner consistent with unpredictable returns, giving prey little additional information about risk.
But why wouldn't Sharp-shinned Hawks focus their hunting on the areas with the most potential prey bird feeders? One possibility is that behaviorally responsive prey diminish the "hotspot" quality of feeders. Although feeder hotspots are sources of abundant prey, the individuals at such feeders generally benefit from group vigilance as a result of these higher densities.
As a result, the vulnerability of the prey may actually be lower at feeders than at other locations. In addition, unpredictable movement may reflect a sort of "prey management" by predators, whereby predators spread their hunting activity over multiple areas in an effort to avoid inflating the antipredator behavior of their prey.
This hunting strategy may be effective when prey are anchored to high-resource areas such as feeders and use antipredator behaviors, such as high vigilance, that reduce a predator's attack success if it attacks frequently and predictably. Seabirds are choking on ocean plastic video. The tongues of cormorants and other fish-eating species are small because these species swallow prey whole and tongues are not needed to manipulate or position food in the oral cavity. Dorsal view of the surface of the lower bill of a Great Cormorant Phalacrocorax carbo.
Arrow shows the tongue with sharpened tip. Scale bar, 12 mm. Lateral view of the cormorant tongue. The tongue and the small anterior and posterior areas of the mucosa of the bill are covered by white keratinized epithelium. Black arrow shows short base of the tongue. White arrow shows the median crest on the dorsal surface of the tongue. A, anterior; B, posterior. Scale bar, 3 mm Source: Detailed view of the horny tip left of the Guadeloupe Woodpecker tongue in vivo position Villard and Cuisin Dorsal view of the tongue of the Spotted Nutcracker Nucifraga caryocatactes.
Arrows show two elongated processes of the apex. A, apex, B, body, R, root, LP, laryngeal prominence. Scale bar, 3 mm. Lateral view of the tongue of the nutcracker. Arrow shows elongated processes, pointed diagonally, B, body, R, root. Hummingbird tongues are fluid traps, not capillary tubes -- Hummingbird tongues pick up a liquid, calorie-dense food that cannot be grasped, a physical challenge that has long inspired the study of nectar-transport mechanics.
Existing biophysical models predict optimal hummingbird foraging on the basis of equations that assume that fluid rises through the tongue in the same way as through capillary tubes. Rico-Guevara and Rubega found that hummingbird tongues do not function like a pair of tiny, static tubes drawing up floral nectar via capillary action. Instead, the tongue tip is a dynamic liquid-trapping device that dynamically traps nectar by rapidly changing their shape during feeding.
In addition, the tongue—fluid interactions are identical in both living and dead birds, demonstrating that this mechanism is a function of the tongue structure itself, and therefore highly efficient because no energy expenditure by the bird is required to drive the opening and closing of the trap.
These results rule out previous conclusions from capillarity-based models of nectar feeding and highlight the necessity of developing a new biophysical model for nectar intake in hummingbirds. Hummingbird tongue tips twist to trap nectar. How the hummingbird tongue really works with videos. Close encounters with possible prey.
You want to live 10—20 years. You are peering under leaves, poking into rolled ones, searching around stems, exploring bark crevices and other insect hiding places. Abruptly an eye appears, 1—5 cm from your bill. The eye or a portion of it is half seen, obstructed, shadowed, partly out of focus, more or less round, multicolored, and perhaps moving. Now, a safe few meters away, are you going to go back to see whether that was food?
Associated body patterns often suggest other head and facial features, which in turn enhance the eye-like nature of the spots. None of these patterns exactly matches the eyes or face of any particular species of predator; but, even when quickly and partially glimpsed, all give the illusion of an eye or face. These false eyes are mimicking the eyes and faces of such predators of insect-eating birds as snakes, lizards, other birds, and small mammals, as perceived at close range by the insectivorous birds in their natural world.
Note the distended throat of this American Kestrel.